Method of modifying the surface of a solid polymer substrate and the product obtained

ABSTRACT

A method of modifying the surface of a solid polymer substrate comprising the steps of a) generating radicals on the substrate surface by subjecting it to a gas plasma or by subjecting it to UV light, and b) treating the surface with a vapor of a monomer or a monomer mixture comprising cyano acrylate and/or isocyanate, where step b) starts before step a), simultaneously with step a), under step a), or follows immediately after step a), and a polymer substrate modified accordingly; a method of binding an organic binder material to a surface of a solid polymer substrate comprising the steps of modifying the surface of the substrate by said method, and bringing the organic material in contact with the surface of the substrate, and a polymer bonded to an organic material by the last mentioned method.

The present invention relates to a method of modifying the surface of asolid polymer substrate and the product obtained.

Untreated or unmodified polymer substrates are in general difficult topaint, print on or adhere to. Particularly, it is difficult to achieve abonding between an untreated polymer surface and an organic top layer.

It is well-known in the art to clean the surface of a polymer substrateor to modify the surface of the substrate in order to change the surfacecharacteristics of the substrate and thereby improve the affinitybetween the substrate surface and an organic binder.

The surface treatments normally used are flaming, chemical treatmentwith chrome sulphuric acid, or corona treatment. Flaming is the simplestmethod. In this method the substrate surface is stroked by a flame. Boththe flaming treatment and the chemical treatment are very rough methodswhich may weaken the cohesiveness of the polymer substrate. Further, itis normally preferred to avoid the use of strong acids in theproduction. One of the problems of using flame treatment is that themethod is not controllable. Often the surface of the polymer is severelydegraded due to this treatment.

One of the most conventional methods is corona treatment. By use of thismethod a number up small sparks are created on the substrate surface (asilent discharge). These sparks often result in an erosion of thesurface as well in a generation of free radicals. The free radicals arenormally terminated by reaction with oxygen.

The article by K. Johnsen et. al. “Modification of Polyolefin Surfacesby Plasma-Induced Grafting” Journal of Applied Polymer Science, Vol. 59,pp. 1651-1657 (1996), describes a method of modifying polyolefinsurfaces by grafting polar monomers onto the surface. The grafting isinitiated by a treatment of a LD-PE substrate surface with argon plasmafor 5 minutes, whereby free radicals are generated. Thereafter, orsimultaneously, the surface is treated with one of the followingmonomers: Acrylic acid, glycidyl methacrylate, 2-hydroxy ethylacrylateand methyl acrylate. The grafting time was from 7 to more than 90minutes. The contact angle against water for the (acrylic acid monomer)treated LD-PE surface was improved from about 91.5° to about 38°.However, it was not tested if the affinity between the treated LD-PEsurface and an organic binder actually was improved. It was finallyconcluded that the method is too slow to be of direct industrialapplication.

A similar method of plasma treatment of PTFE surfaces is described in JPpatent application No. 55-131026. A PTFE surface was treated with a Hegas plasma for 2-10 minutes and simultaneously treated withN,N-dimethylaniline, N-monomethylaniline, aniline, benzonitrile,benzamide or pyridine monomers. An oxygen plasma treatment of PTFE wasconducted without monomer treatment, and peel tests of the PTFE samplesshowed that there was no significant difference in peel resistancebetween the PTFE surfaces that were treated with monomers and the PTFEsurfaces that were only treated with oxygen plasma. Actually, the latterhad a better peel resistance than the monomer treated surfaces.

A method of electroless plating is described in JP application No.92-240189. In this method a PTFE substrate is irradiated with UV laserin the presence of amine or amide followed by an immersing in anelectroless metal plating solvent. This result in a good bonding of theplated metal to the substrate. However, this method can only be used foradhering thin metal layers to polymer substrates.

Another method for treatment of a fluorinated polymer is described in JPpatent application No. 90-081728. In this method a polymerisable monomerselected between acrylic acid, styrene sulphonic acid and acrylamide isgrafted onto the surface of the resin by coating the surface with amonomer solvent, followed by irradiation of the surface with UV laser.

The object of the present invention is to provide a industriallyapplicable method of binding a polymer substrate to an organic bindermaterial, which method results in an improved binding strength.

More specifically the object of the present invention is to provide anindustrially applicable method of modifying a polymer surface, by use ofwhich method the surface affinity against organic binder material isimproved.

Another object of the present invention is to provide a fast method ofmodifying a polymer surface, by use of which method the surface affinityagainst organic binder material is improved.

A further object of the present invention is to provide a method ofmodifying a polymer surface, which does not result in any severedepolymerization of the polymer substrate, in particular when thesubstrate material exhibits fluorine and/or tertiary carbon atoms, andby use of which method the surface affinity against organic bindermaterial is improved.

These objects are achieved by a method of modifying a surface of a solidpolymer substrate comprising the steps of

-   a) generating radicals on the substrate surface by subjecting it to    a gas plasma or by subjecting it to UV light, and-   b) treating the surface with a vapour of a monomer or a monomer    mixture comprising cyano acrylate and/or isocyanate,    where step b) starts before step a), simultaneously with step a),    under step a), or follows immediately after step a).

Polymer surface treatment with plasma or UV light for other purposes iswell known in the art. Plasma treatment of polymer surfaces is e.g. usedfor cleaning purpose. In this known method the generated radicals areoften terminated by reaction with oxygen.

The polymer substrate can be of any polymer material provided that freeradicals are created on the surface of the material when it is subjectedto a gas plasma and/or treated with UV light. Preferably the polymersubstrate is a silicon rubber, a polyolefin, or another thermoplastic.

The thermoplastic is preferably selected betweenpolytetra-fluoroethylene (PTFE),tetra-fluoroethylene-hexa-fluoropropylene-copolymer (FEP),polyvinyl-difluoride (PVDF), polyamides, such as e.g. nylon 6.6 andnylon 11, and polyvinyl-chloride (PVC). The polyolefin is preferablypolypropylene (PP) or poly (4-methyl-1-pentene) (PMP)

The substrate can have any shape and any size. Preferably the polymersubstrate is in the form of film, sheet, pipe, rod, porous or non-porousbody, fabric, non-woven fabric, fibres or threads. In one very preferredembodiment the polymer substrate is produced by injection moulding.

When the generation of radicals on the substrate surface is obtained bysubjecting the substrate to UV light, the wavelength and the intensityof the UV light are selected depending on the constitution of thepolymer. A skilled person can by use of ordinary techniques optimise themethod by selecting wavelength and intensity of the UV light as well asselecting the time of radiation. The time of radiation should naturallybe sufficiently long to create the radicals on the surface. On the otherhand, the time of radiation should not be too long, as this might resultin degradation of the substrate.

The generation of radicals on the substrate surface is preferablyobtained by subjecting the substrate to a gas plasma. The plasma can begenerated by any known methods, but preferably the gas plasma isgenerated by excitation of a gas in a direct current (DC), audiofrequency (AF), radio frequency (RF) or microwave (MW) generatedelectric field. Most preferably the gas plasma is generated byexcitation of a gas in a direct current (DC) or by exitation using radiofrequency (RF).

The intensity of the used gas plasma should preferably have a levelensuring creation of radicals in the polymer surface. If the level istoo high, this may result in severe damage of the bulk-polymer(depolymerization). Hence, the powerlevel of the plasma should beoptimized so that surface radicals are created, but no serious damage ismade to the bulk.

A preferred method of generating a plasma is described in theapplicant's European patent application No. EP 96610018.2. In thismethod gas is subjected to an electric field generated by an electrodesystem comprising n electrodes, n being an integer greater than or equalto 3, preferably between 3 and 30, each of the n electrodes beingconnected to one of the following AC voltages:

-   -   U_(r)(t)=U₀sin(2π f·t)    -   U_(s)(t)=U₀sin(2π f·t+2π/3)    -   U_(t)(t)=U₀sin(2π f·t−2π/3)

-   where f is a frequency in the range from 10 to 10000 Hz, preferably    from 30 to 200 Hz, more preferably from 50 to 60 Hz, U₀ is a voltage    in the range from 50 to 10000 V,

-   at least one electrode being connected to U_(r), at least one    electrode being connected to U_(s), and at least one electrode being    connected to U_(t). The electrode is preferably placed in a circle.

The gas can be any inert gas or mixtures thereof, preferably a gasselected between He, Ne, Ar and Kr.

By the term “inert gas” is meant a gas that does not react chemicallywith the polymer surface.

The monomer or monomer mixture preferably comprises one or more of 2-C₁-C₁₀ alkyl cyano acrylate and diisocyanate, and more preferably one ormore of 2-ethyl cyano acrylate and toluene 2,4-diisocyanate. The monomermixture may also comprise one or more of acrylic acid, methyl acrylate,2-hydroxy-ethylacrylate, N-ethyl-2-methyl allylamine, glycidylmethacrylate, diallylamine, and/or other vinyl group containingmonomers.

More preferably the monomer vapour comprises 60 mole % or more of2-ethyl cyano acrylate vapour, most preferably 90 mole % or more of2-ethyl cyano acrylate vapour.

In a particularly preferred embodiment of the invention the monomermixture prior to vaporization consists essentially of a mixture of2-ethyl cyano acrylate, a water free acid preferably having a partialvapour pressure which is lower than the partial vapour pressure of2-ethyl cyano acrylate, and up to 40 weight-% of another filler,preferably a mixture of 2-ethyl cyano acrylate and an acid having apartial vapour pressure in the plasma which is lower than half thepartial vapour pressure of 2-ethyl cyano acrylate, most preferably amixture of 2-ethyl cyano acrylate and a polyphosphoric acid. By “filler”is meant a material which does not act as a monomer under the treatmentconditions. Even more preferred, the monomer mixture prior tovaporization consists of a mixture of 60 to 97 weight-% of 2-ethyl cyanoacrylate, up to 10 weight-% of polyphosphoric acid and up to 40 weight-%of another filler.

The method is preferably carried out in a reactor which may be at leastpartly evacuated from air and water vapour. Further, the reactor shouldhave a channel for feeding the inert gas, and a channel for feeding themonomer or monomer mixture. The monomer or monomer mixture is introducedas a gas, e.g. by evaporation from a bottle or by injection, e.g.through a nozzle.

In a preferred embodiment of the invention, the substrate is placed inthe reactor, and some or all of the air and optionally water vapour areevacuated.

Inert gas is fed into the reactor, and the plasma is generated (stepa)). Before, simultaneously or shortly thereafter monomer or monomermixture is fed into the reactor (step b)).

The generation step a) is preferably carried out for a period of between0,01 0.01 and 1000 seconds and the treatment step b) is preferablycarried out for a period of between 0,1 0.1 and 1000 seconds.

The generation step a) is preferably carried out for a period which isequal to or longer than the period of the treatment step b). Thetreatment step b) may continue when step a) has ended, even though thereno longer is generated radicals on the substrate surface. Thiscontinuation of step b) will then result in a polymerisation of monomersonto the monomers which already have been bound to the polymer surface.

If the surface of the polymer substrate is contaminated with water, oilor other organic contaminants, step a) is preferably carried out formore than 30 seconds, and step b) is started 10 to 30 seconds after stepa) so as to clean the surface before the monomers are polymerized ontothe substrate surface. Step a) and b) are preferably endedsimultaneously.

The partial pressure of the inert gas or the plasma in step a) ispreferably between 0.1 and 10000 Pa.

The monomer pressure in step b) is preferably between 0,1 0.1 and 100000Pa, more preferably between 10 and 1000 Pa.

The total pressure, i.e. the sum of the partial pressures of the air,optionally water vapour, the inert gas or plasma and the monomer, understep a) is preferably equal to the total pressure under step b), thetotal pressure is preferably between 0,2 0.2 and 100000 Pa, morepreferably between 0,2 0.2 and 10000 Pa, and most preferably between 10and 1000 Pa.

The temperature is not so important and should preferably be the sameunder both step a) and step b). Normally the temperature will rise alittle under the generation step a). Preferably the temperature underboth step a) and step b) is between 250 and 450 K, most preferablybetween 280 and 330 K.

After having modified the substrate surface as described above andthereby improved its affinity against organic binder material, thesubstrate and such an organic binder material may be bonded to eachother to create a strong bonding.

The present invention therefore also concerns a method of binding anorganic binder material to a surface of a solid polymer substrate. Thismethod comprises the steps of

-   i) modifying the surface of the substrate by the method as described    above and-   ii) bringing the organic material in contact with the surface of the    substrate.

Preferably the modifying step i) is carried out not more than 24 hoursbefore the contacting step ii), most preferably the modifying step i) iscarried out between immediately before and 1 hour before the contactingstep ii).

The organic binder material may be any organic material which is eithersolid or is able to solidify e.g. by evaporation of a solvent, or by achemical hardening. Preferred binder material is a paint, an adhesive oranother polymer material, preferably selected between a crosslinkablethermoplastic and a crosslinkable rubber.

The organic binder material may as well be a substrate which may also besurface modified by the method defined in claims 1 to 15 herein.

In a particularly preferred embodiment the organic binder material issubjected to a plasma treatment immediately. before the contacting stepii).

The invention also relates to the polymer substrate modified accordingto the method as defined in claims 1 to 15 herein as well as the polymerbonded to an organic material by the method as defined in claims 16 and17 herein.

Further the invention relates to a polymer composite material as definedin claim 20 herein, wherein the polymer substrate is a polymer fiber, apolymer thread or a polymer filler and the organic material is anotherpolymer, preferably a polyester or an epoxy polymer.

Experimental system

All pretreatments were carried out in an experimental plasma system,essentially consisting of a 22 liter vacuum vessel having two inletchannels for gas and for monomer, respectively, and an electrodearrangement as described above with reference to the applicant's EPapplication No. 96610018.2 The plasma is generated using the 3-phasetechnique described above with a frequency of 50 Hz and U₀ of 240 to280V. 27 electrodes are arranged in a circle around the vacuum vesselwith a diameter of 17 cm. Every third of the electrodes is connected toU_(r)(t), another every third of the electrodes is connected toU_(s)(t), and the last every third of the electrodes is connected toU_(t)(t).

The electrodes are made from aluminium rods, having a diameter of 20 mmand a length of 30 cm. When describing the voltage between theelectrodes, reference is made to the voltages U_(r), U_(s) and U_(t) asdescribed above.

All pretreatments were carried out by placing the samples in the centreof the vacuum-vessel. The chamber is evacuated by use of an Edwards EH250 Roots-blower, backed by an Alcatel rotary vane pump. The pressure ismonitored using an Alcatel pirani-gauge, mounted on the start of thepumping-line.

The chamber of the vacuum-vessel comprises as mentioned a channelthrough which it is possible to evaporate 2-ethyl cyano acrylate intothe plasma system.

EXAMPLE 1

Pretreatment of polytetraflouroethylene (PTFE)

These experiments were carried out on PTFE strips (Pampos, Germany)having a length of 100 mm, a thickness of 2 mm, and a width of 10 mm.

To test the effect of the pretreatment, the strips were after treatmentbonded to an aluminium strip using an epoxy-based adhesive (AralditRapid). This test was carried out according to the ISO 4587 test formeasurement of the shear-strength of adhesives.

The following scheme was followed:

A flow of 2 sccm Ar is established, and pumping is carried out until asteady-state situation is reached. After this, the plasma is ignited ata voltage of 280 V. After 20 s the 2-ethyl cyano acrylate containingbottle is opened, and the system is run for a further 30 s. Aftertreatment the PTFE-strips were glued to sand-blasted Al-strips using theadhesive mentioned above.

The shear-strength of the bond was measured to be in excess of 2 N/mm²,which is considerably higher than the value of 0.1 N/mm which ismeasured for untreated strips.

The same results have been achieved using a PUR-based adhesive.

EXAMPLE 2

Pretreatment of polypropylene (PP)

These experiments were carried out on pure PP (Hostalen PP fromHoechst). The samples were injection moulded pieces having dimensions of75×25×1 mm. To test the increased adhesion, the PP pieces were afterpretreatment painted using a solvent based acrylic paint (Motip AcrylLack-Spray, Motip BV, Postbus 221, 8330 AE Steenwijk, Holland). The testwas carried out according to the ISO 2409 international standard. Priorto the pretreatment the surfaces were washed in hexane and dried for 3hours.

The following scheme was followed:

2 sccm Ar flow is established, and pumping is carried out until asteady-state situation is reached at a measured pressure of 10 pa. Bythe term “steady-state” is meant that Ar is pumped into the chamber andout again until the chamber is substantially free of air and a constantpressure is reached.

After a steady-state situation is reached, the 3-phase plasma is ignitedat a voltage of 280V. After 10 s the voltage is reduced to 240 V, andthe valve to the 2-ethyl cyano acrylate bottle (Bison Super glue) isopened. After further 30 s the voltage is reduced to 0V. After further20 s the 2-ethyl cyano acrylate bottle is closed, and air is let intothe vacuum vessel.

Approximately 1 hour after the pretreatment, the sample is painted usingthe paint mentioned above.

When testing painted PP, painted and pretreated according to theprocedure outlined above, the test result obtained according to the ISO2409 standard is determined to be “0-1”.

When following the same procedure on untreated, painted polymer, thetest result is determined to be “5”.

The test results are determined in accordance with ISO 2409 bysubjecting the material to a peel test and visually determining howstrong the bonding between the substrate and the paint is. The lower thecharacter, the better the bonding.

Improved adhesion may be achieved by using monomers other thanethyl-cyano-acrylate. To exemplify this, experiments similar to thosedescribed in example 1 and 2 were repeated using 2,4-diisocyanateinstead of 2-ethyl-cyanoacrylate. The monomer used in the followingexamples was a commercial grade Aldrich 21,683-6 (80% toluene2,4-diisocyanate, 20% 2,6 toluene 2,4-diisocyanate).

The results were similar to the results obtained in example 1 and 2,respectively.

EXAMPLE 3

The procedure for pretreatment was exactly as described in example 1.The only difference is that the ethyl cyanoacrylate in the vaporizer wassubstituted with the compound mentioned above. In this, later experimentthe tensile strength of the bonding and not the shear strength was usedto quantizize the improvement. The tensile strength of the bondingbetween untreated PTFE and epoxy was below the resolution of the usedtest equipment. The tensile strength of the bond between treated PTFEand epoxy was measured to be 7.6 N/mm². Failure of the bonding was dueto cohesive failure in the PTFE material itself, and not due todebonding between PTFE and epoxy.

EXAMPLE 4

Polypropylene (PP) was treated according to the procedure described inexample 2. The only difference is that the ethyl cyanoacrylate in thevaporizer was substituted with the compound mentioned above. In this,later experiment the improvement of bondstrength was quantifiedaccording to the procedure described in example 3. The measured tensilestrength in this experiment was 14.8 N/mm² i.e. comparable to thetheoretical strength of the used epoxy adhesive. No tests were carriedout on non-treated PP samples, as it is known that it is not possible toachieve a bond between PP and epoxy.

EXAMPLE 5

The surface of injection moulded glas glass fibre reinforced polyamide6.6 curved test specimens was modified as described in example 1. Sevendays after this treatment the test specimens were painted withpolyurethane lacquer: Alexit 341-83, 401-83, 401-54 and 412-00 fromMankiewicz GmbH, Hamburg).

Thereafter the test specimens were tested according to DIN 53 151(lattice cutting with subsequent fixing and peeling of Tesa 4122). Scalefrom Gt0 to Gt5 (Gt0: no peeling). Hereby the following results wereobtained:

lattice cutting +Tcsa 4122 Alexit 341-83 Gt0 Gt0 Alexit 401-83, 401-54,Gt0 Gt0 412-00

The same result was obtained after ageing in a moist closet at 60° C.for 48 hours.

1. A method of modifying the surface of a solid polymer substratecomprising the steps of a) generating radicals on the substrate surfaceby subjecting it to a gas plasma or by subjecting it to ultravioletlight, and b) treating the surface with a vapour of a monomer or amonomer mixture comprising cyano acrylate and/or isocyanate, where stepb) starts before step a), simultaneously with step a), during step a),or follows immediately after step a).
 2. A method according to claim 1,wherein the monomer or monomer mixture comprises one or more of C₁-C₁₀2-alkyl cyano acrylate and diisocyanate.
 3. A method according to claim2, wherein the diisocyanate is toluene 2,4-diisocyanate.
 4. A methodaccording to claim 1 wherein step b) comprises treatment of the surfacewith a monomer mixture comprising acrylic acid, methyl acrylate,2-hydroxy-ethylacrylate, N-ethyl-2-methyl allylamine, glycidylmethacrylate, diallylamine, and/or other vinyl group containingmonomers.
 5. A method according to claim 1, wherein step b) comprisestreatment of the surface with a monomer vapour comprising 60 mole % ormore of 2-ethyl cyano acrylate vapour.
 6. A method according to claim 5,wherein step b) comprises treatment of the surface with a monomer vapourcomprising 90 mole % or more of 2-ethyl cyano acrylate vapour.
 7. Amethod according to claim 1, wherein the monomer mixture prior to thevaporization consists essentially of 2-ethyl cyano acrylate, an acidhaving a partial vapour pressure in the plasma which is lower than thepartial vapour pressure of 2-ethyl cyano acrylate, and up to 40 weight-%of another filler.
 8. A method according to claim 7, wherein the acid isa polyphosphoric acid and is present prior to the vaporization in aconcentration up to 10 weight-%.
 9. A method according to claim 1,wherein the polymer substrate is a silicone rubber or a thermoplastic.10. A method according to claim 9, wherein the polymer substrate isselected from the group consisting of a polytetra fluoroethylene (PTFE),tetra fluoroethylene hexa fluoropropylene copolymer (FEP), polyvinyldifluoride (PVDF), nylon 6,6, nylon 11, and polyvinyl chloride (PVC).11. A method according to claim 1, wherein the polymer substrate is apolyolefin.
 12. A method according to claim 1, wherein the polymersubstrate is an injection moulded polymer component, a polymer fiber, apolymer thread or a polymer filler.
 13. A method according to claim 1,wherein the generation of radicals on the substrate surface is obtainedby subjecting the substrate to a gas plasma, and the gas is an inertgas.
 14. A method according to claim 1, wherein step a) comprises thestep of generating radicals by use of a gas plasma generated byexcitation of the gas in a direct current (DC), audio frequency (Ar),radio frequency or microwave generated electric field.
 15. A methodaccording to claim 1, wherein the monomer pressure in step b) is between0.1 and 100000 Pa.
 16. A method according to claim 15, wherein step a)is carried out for more than 30 seconds, and step b) is started 10 to 30seconds after step a).
 17. A method according to claim 1, wherein thegeneration step a) is carried out for a period of between 0.01 and 1000seconds, and the treatment step b) is carried out for a period ofbetween 0.1 and 1000 seconds.
 18. A method according to claim 1, whereinthe temperature is the same under both step a) and step b).
 19. A methodaccording to claim 18, wherein the temperature under both step a) andstep b) is between 250 and 450 K.
 20. A method according to claim 1,wherein the total pressure under step a) is equal to the total pressureunder step b).
 21. A method according to claim 20, wherein the totalpressure under step a) and step b) is between 0.2 and 10000 Pa.
 22. Apolymer substrate modified according to claim
 1. 23. A method of bindingan organic binder material to a surface of a solid polymer substratecomprising the steps of modifying the surface of the substrate by themethod according to claim 1, and bringing the organic material incontact with the surface of the substrate.
 24. A method according toclaim 23, herein wherein the organic binder material is a paint, anadhesive or another polymer material.
 25. A polymer bonded to an organicmaterial by the method according to claim
 23. 26. A polymer compositematerial according to claim 25, wherein the polymer substrate is apolymer fiber, a polymer thread or a polymer filler, and the organicmaterial is another polymer.
 27. A polymer composite material accordingto claim 26, wherein the organic material is a polyester or an epoxypolymer.